TITLE: GCN CIRCULAR NUMBER: 22031 SUBJECT: GRB 171020A: Confirmation of OT fading DATE: 17/10/21 03:32:12 GMT FROM: Moskvitin Alexander at SAO RAS A. Moskvitin (SAO RAS), O. Spiridonova (SAO RAS), A. Volnova (IKI RAS) We observed the field of GRB 171020A (Page et al., GCN 22028) with the 1-m telescope of SAO RAS, Zeiss-1000 on 2017.10.20, starting at 23:18 UT, i.e. ~11 minutes after the trigger. The OT (Malesani et al., GCN 22029) is clearly detected in the individual 300 sec. frames. The coordinates of OT: R. A. = 02:36:59.62; Dec. = +15:12:15.6 (J2000). Date UT start t - T0 Filter Exp. OT Err. UL (mid, days) (s) 2017.10.20 23:18:05 0.02016 Rc 6*300 20.9 +/- 0.1 22.3 23:55:27 0.04827 Rc 7*300 21.5 +/- 0.1 22.5 The temporal decay slope is about -0.3, F ~ (t-T0)^alpha. The calibration were made against the nearby USNO-B1.0 stars (R2 magnitudes): 1051-0026883 02:37:00.38 +15:11:09.6 18.59 1052-0027787 02:36:57.49 +15:12:36.8 16.62 1052-0027776 02:36:50.26 +15:12:39.8 18.15